A magnet assembly for use in carrying out nuclear magnetic resonance experiments on a body or sample. The assembly comprises a set of superconducting coils (12) within a cryostat, located about a bore, and arranged to generate a substantially uniform magnetic field in a primary working volume (101) within the bore, and to generate a substantially uniform magnetic field in a secondary working volume (100) within the coil structure and separate from the bore. At least part of a hyperpolarisation system intersects the at least one secondary working volume (101) so as to hold a sample to be hyperpolarised in the secondary working volume.